FCCL (Flexible Copper Clad Laminate) is a copper-clad laminate having a better flexural property formed by bonding a single side or double sides of a flexible insulating material such as polyester films (PET) or polyimide films (PI) to copper foil having a certain thickness by a certain technological treatment. FCCL comprises 3L-FCCL containing adhesives and 2L-FCCL containing no adhesives. As compared with 3L-FCCL, 2L-FCCL has better performances in, such as thermal resistance, size, stability, ageing resistance, reliability and the like because of containing no epoxy adhesives or acrylate adhesives, so as to achieve rapid development.
At presence, there are mainly three processes for preparing 2L-FCCL, i.e. (1) Coating (Casting), (2) Lamination, and (3) Sputtering/Plating, having their own peculiarities.
1. Casting:
The early casting process comprising single-side coating a prepolymer (polyamic acid, PAA) of PI onto the surface of copper foil, drying to remove the solvent and high temperature imidizing. Such process has relatively simple procedures and is easy to carry out, but the product has a worse bonding property between the copper layer and PI and a worse size stability.
In recent years, a multilayer coating process is developed in the industry, comprising coating (casting) on the surface of the copper foil a layer of thermoplastic PI (TPI) resin, and then a layer of PI (low CTE-PI) resin having a low thermal expansion coefficient, finally a layer of thermoplastic PI(TPI) resin, high temperature imidizing and laminating with the copper foil to form a product.
The multilayer coating (casting) process may achieve the effects at two aspects, i.e. (1) having good consistency between the bonding property and size stability of the product; and (2) maintaining the symmetry of the whole structure and reducing the curling of the product.
There are a few flaws with the multilayer coating (casting) process, such as complex procedures, higher device investment coast, and the like. Due to the technical problems during the preparation of thin copper foil, such process is difficult to be used for producing 2L-FCCL having a copper foil thickness of 18 μm or less. Such process is mainly used for producing single sided laminates.
2. Lamination is one technological process which has been rapidly development in recent years, comprising double-side or single-side coating a prepolymer (polyamic acid, PAA) of a thermoplastic PI resin having excellent bonding properties on the basis of PI (CTE-PI) film having a low expansion coefficient, drying and imidizing to form a composite film, re-melting the thermoplastic resin at high temperature and high pressure, laminating with copper foils to form a single-sided or double-sided product.
Currently, the basement membrane providers provide such composite films consisting of PI films having a high size stability and a layer of thermoplastic PI resin (TPI) coated on the PI films. FCCL manufacturers can directly hot-press the composite films and copper foils into laminates.
Lamination is characterized in (1) relatively simple production procedures, a higher cost than the coating process; (2) being adapted to the product model of small batch and various variety; (3) being useful for production of single sided and double sided products; and (4) the product having the comprehensive performances of excellent bonding properties and size stability.
Due to the technical problems during the preparation of thin copper foil, such process is also difficult to be used for producing 2L-FCCL having a copper foil thickness of 18 μm or less.
3. Sputtering/Plating
In vacuum environment, the ionized argon ion (Ar+) is used to bombard the target (copper) surface, so that atoms (copper atoms) on the target are “sputtered” (there will be spray sputtering if stones are thrown into water). The sputtered copper atoms are adsorbed and deposited onto the surface of the substrate PI films to form a thin copper layer, and then the copper layer is thickened by electroplating to the required thickness.
By such process, various single sided or double sided 2L-FCCLs having a thickness of 5-12 μm will be readily produced, but the peeling strength of copper foils is greatly lower than 2L-FCCLs produced by the coating process and lamination process.
Among said three processes, 2L-FCCLs produced by the coating and lamination processes have a better bonding force between copper foils and PI matrix, i.e. a high peeling strength, but have a higher cost due to complex production processes and high requirements on the devices. Furthermore, since the formed copper foils (rolled copper foil and electrodeposited copper foil) shall be used, rolled copper foil and electrodeposited copper foil are difficult to have a thickness less than 19 μm, such as 12 μm, 9 μm, 7 μm and the like due to the technological limitations. Thus the application thereof in high-grade precision electronic products based on HDI (high density internet base plate) and COF (Chip on Flex), such as liquid crystal (plasma) displays, liquid crystal (plasma) televisions and the like, is limited.
On the contrary, the sputtering/plating can be used to readily produce 2L-FCCLs of various ultrathin copper foils (such as 12 μm, 9 μm, or even 7 μm and 5 μm) with a lower cost. However, because of limitation of the sputtering technologies, the bonding force between the metal copper foils and PI films (peeling strength) is far lower than the bonding force between the metal layer and films in 2L-FCCLs produced by the coating and lamination processes, so as to affect the generalization and application thereof. For example, FCCLs produced by the lamination and coating processes have a peeling strength of greater than or equal to 0.6 N/mm, while those produced by the sputtering/plating (spraying plating method) have a peeling strength of about 0.35 N/mm. Thus there really needs a novel process for producing 2L-FCCLs in the art so as to meet the requirements on the development of the industry of flexible printed circuit board (FPC).